With a simple circuit topology, and a feature of achieving soft switching within the full load range, the resonant converter is widely used in power supply and other products. The resonant converter has many advantages, but it still has some drawbacks such as narrow output voltage range, in particular, when light load or no load, the output voltage is restricted by the voltage gain thus unable to continue to down-regulate.
Taking LLC resonant DC/DC converter as an example, FIG. 1 shows the basic form of a full-bridge LLC resonance circuit, which uses a frequency conversion modulation and control. Without considering a dead zone, switching tubes S1, S4 and S2, S3 drive complementarily and symmetrically, and the duty ratio is set as 50%. Thus, the output voltage gain M can be expressed as:
                    M        =                                            V              o                                      V              in                                =                      1                                                                                (                                          1                      +                                              1                        k                                            -                                              1                                                  kf                          n                          2                                                                                      )                                    2                                +                                                                            Q                      2                                        ⁡                                          (                                                                        f                          n                                                -                                                  1                                                      f                            n                                                                                              )                                                        2                                                                                        (        1        )            
Where Vo, Vin are output, input voltage respectively, fs is switching frequency, Lr is resonant inductance, Lm is excitation inductance, Cr is resonant capacitance,
            f      r        =          1              2        ⁢        π        ⁢                                            L              r                        ⁢                          C              r                                            ,            f      n        =                  f        s                    f        r              ,      k    =                  L        m                    L        r              ,fr is resonant frequency, fn is normalized frequency, and k is inductance coefficient.
When the resonant converter works under light load or no load, the corresponding switching frequency fs is greater than the resonant frequency fr. It can be seen through analysis of formula 1 that the voltage gain M is in inverse proportion to the switching frequency fs, and in direct proportion to the inductance coefficient k under the above described state. Theoretically, increasing the switching frequency can obtain relatively small voltage gain, but in practical uses, with the influence of the distributed capacitance of the transformer and the switching elements, a resonance point is newly added on the output voltage gain curve, causing that when LLC resonance circuit works under high frequency state, M increases as fs increases; as for the method to obtain a relatively small high-frequency gain through reducing the inductance coefficient k thus making the voltage gain curve become sharp, it will increase the magnetic core loss and the conduction loss, thus decreasing the efficiency.
At present, an intermittent control mode, commonly known as a hiccup mode, is frequently used to solve the problem of narrow output voltage range of a resonant converter. When the expected voltage is subject to the lower limit of voltage gain and can not be output, the method is to drive to start an intermittent working mode, that is to use a fixed switching frequency (such as the maximum switching frequency) to drive the switching tube, and stop driving after a time period, then start it again, repeat such mode constantly. Such control mode can effectively widen the lower limit of the output voltage, but it has a similarly significant drawback: the output voltage ripple is relatively high, and the first tube conducting after the ripple is sealed increases the switching loss of hard conducting, therefore, it is necessary to upgrade the stress of devices including the output capacitor and the switching elements.
It is thus clear that the above described three methods have different drawbacks when extending the output voltage range of a full-bridge LLC resonant converter. Similarly, the above described methods have similar drawbacks when they are used in other resonant converters based on a theoretical analysis.